Nowadays, in (x)GPON systems, the upstream transmission is controlled at OLT (Optical Line Terminal) side, and bandwidth is controlled via DBA (Dynamic Bandwidth Allocation) mechanism. The upstream flow control is also able to be done based on the DBA mechanism. However the downstream doesn't have corresponding flow control mechanism.
The EPON can apply the pause frame to unicast channel for flow control purpose, but (x)GPON doesn't have the similar downstream flow control mechanism yet. Without the downstream flow control in the (x)GPON system, the (x)GPON system expects the ONU (Optical Network Unit) can buffer the traffic when the congestion happens within ONU or when ONU receives the pause frame from the TINT (User Network Interface) port. Normally ONU will buffer about 100 ms for 100 Mbps UNI port (about 3 Mbytes). This will increase the requirements for the size of the memory of ONU and thus increase the power consumption of the memory of ONU.